The chance discovery of a Roman "ghost fleet" buried in mud just outside Pisa has led experts to conclude that the city was built on a lagoon much like an early Venice.

Archaeologists believe that traces of a community dating back to a pre-Roman era, a sort of Etruscan Venice, may lie beneath the ships. The end of the lagoon civilisation may also offer pointers to the fate of modern Venice - the waterways were silted up by violent floods over a long period.

"The situation in Venice is not just similar to that of Pisa, but is practically identical," said Professor Stefano Bruni of the University of Ferrara.

The find first came to light five years ago when a bulldozer involved in work near the San Rossore station on the outskirts of Pisa came across an ancient wooden ship 10 metres below ground. A large archaeological dig, which was started under Professor Bruni's direction, later found four ships dating from various Roman periods.

The number of vessels rose to six, then nine, and finally 21, including what experts believe may be a Roman warship. They date from 200 BC to AD 500. The ships will soon be housed in a new museum in Pisa's old shipyards.

But, equally important, the discovery has caused the entire geography of the area, and its relationship with the rest of the Mediterranean, to be redefined.

Professor Angelo Bottini, an archaeologist, said the digs had not brought to light the existence of a mere port separated from the sea. Rather, they showed there had been a "network of river and maritime landing places, in which the sea and the rivers were in dialogue".

This network included lagoon islands and wetlands where fresh water combined with salt water. "To compare Pisa and Venice is therefore not rash, even if we must exercise caution," he said.

The discovery of the ships also confirmed claims by ancient sources that before Pisa was a Roman city it had been Etruscan and Hellenic.

The extraordinary state of preservation of the ships was due to what Professor Bottini called flood "trauma". "[Floods] deposited sand in such a violent way that it didn't have time to oxidise the wood," he said.

While this had preserved the ships, it also meant that the wood, when exposed to the air, had to be rehydrated to stop it falling apart.